This invention relates to plasticating extruders for preparing a plasticated material from solids introduced into the extruder, the latter having a screw placed into an elongate barrel and including the following functional sections; a feed section, a transition section and a metering section in a downstream direction.
More particularly, the present invention relates to extruders with a screw having primary and secondary flights providing high melting performance and extrudate stability. Such extruders are disclosed, for example, in U.S. Pat. Nos. 3,897,938; Re. 28,901; 4,405,239 and 4,472,059, the latter being the closest.
The screw of the last patent has the following flights:
two primary flights passing through the transition section and defining channels for continuous conveying the material in the downstream direction as the material is melted within the channels, each said flight having a leading face facing downstream and a trailing face facing upstream, and each said channel extending between the leading and trailing faces, the primary flights in the transition section being opposite one another about a longitudinal axis of the screw such that radial forces conveyed to the material in the channels of the transition section are substantially balanced;
two secondary flights, each of them being associated with one of said primary flights for segregating the already plasticized material from the not-plasticated-yet material in the corresponding channel, the secondary flights being located only in the transition section and being opposite one another about the longitudinal axis such that radial forces conveyed to the material in the transition section are substantially balanced.
As a result of having balanced flights, lateral deflection, wear, vibration and galling of the screw and barrel are substantially reduced.
However, the screw of the last patent has also two flights in the feed section. This decreases the volume available for accepting the raw material, not plasticized yet and, therefore, having density much lower than that of the plasticized material in the downstream sections. That is why the flow of the material in the feed section is reduced, and the screw has thus the lower carrying capacity which is not matched by the subsequent sections.